It is known in an internal combustion engine to employ cam profile switching (CPS) to enhance engine fuel economy at low engine speeds whilst ensuring increased maximum power at high engine speeds. Switching between two different cam profiles permits an engine to operate with small inlet valve lift at relatively low engine speeds and a larger inlet valve lift at relatively high engine speeds.
Any degradation of the CPS system may cause an effect on tailpipe emissions and as such there is a legislative requirement that the engine management system (EMS) monitors the performance of the CPS system and reports any system degradation to the driver.
U.S. Patent application 2005/0204805 and U.S. Pat. No. 6,213,068 describe methods which make use of exhaust air-fuel ratio (AFR) as a means of detecting correct system operation. U.S. 2005/0204805 discloses a system that continuously assesses the AFR and attributes an AFR at a given time period with a specific cylinder. It determines that a degradation exists if the AFR of a specific cylinder departs from that which would be anticipated for it, e.g., if its value is significantly different from others.
The system disclosed in U.S. Pat No. 6,213,068 continuously monitors inlet air flow (mass and pressure) and compares these values against values anticipated from algorithms based on engine speed and throttle position. If degradation is perceived then bank fuelling compensation values are considered in order to identify which bank is likely to have degraded.
These known methods allow for the detection of a single tappet degradation but may be unreliable and/or difficult to calibrate and may be insensitive to multiple tappet degradations. A monitor that will only detect bank degradation would be simpler and therefore potentially more reliable and easier to calibrate.
A typical CPS system comprises a variable lift tappet on each inlet valve of each cylinder (two per cylinder). A tappet will operate on the high lift cam profile if high pressure oil feed is supplied to the side of the tappet. Therefore, a CPS system also includes electronically-activated solenoid switches which, when actuated, will supply oil pressure to all tappets. On engines having a V configuration, one solenoid per bank is provided.
Automobile engines with a V layout are usually equipped with one exhaust pipe per bank. Each exhaust bank has its own air-fuel ratio sensor. Fueling is independently controlled in each bank to ensure that lambda is maintained near to a value of 1 in order to ensure efficient exhaust catalyst performance and thus efficient tailpipe emissions. At certain engine operating conditions i.e. certain engine speed, load, variable valve timing settings, a change in the cam profile switching position from one setting to another will cause a change in the air flowing through the engine. If the cam profile switching position is switched on one bank alone, one bank will accept less air than it did prior to the switch and the other, more air than it did prior to the switch. As a result, a cam profile switching event can manifest itself as a deflection in the air-fuel ratio of each bank.
There is typically a time lag between initiation of a cam profile change, (i.e. applying a switching signal to a solenoid switch) and a resulting change in the air-fuel ratio of either exhaust bank. This is because of the air induction and combustion process. That is, the change in cam profile initially restricts the intake air flow into the switched bank (and therefore increases it in the other bank). This altered intake air quantity will take a finite time to progress through the cylinders and combustion process before being exhausted and causing a fluctuation in the air-fuel ratio readings.
Hence, once a switching signal has been applied to the solenoids, it can be anticipated that at some later stage a deflection in the air-fuel ratio will ensue, provided that the switching process is functioning as desired. However, if there is degradation and the application of the signal to the solenoid switches does not result in a switch to a different cam profile, then there will be no anticipated deflection measurable in the exhaust air-fuel ratio.
According to a first aspect, the description consists of a method for detecting degradation in a cam profile switching system of an internal combustion engine having a first cylinder bank and a second cylinder bank, the method comprising: indicating degradation of a cam profile switching device as the output of a first sensor located in the exhaust path of said first cylinder bank varies from the output of a second sensor located in the exhaust path of said second cylinder bank.
Hence, if the relationship between the air-fuel ratios of each bank remains substantially unchanged after a cam profile switching signal has been applied, then this indicates that the switching process must have been unsuccessful.
According to a second aspect, the description consists of a method for detecting a degradation in a cam profile switching system of an internal combustion engine having a plurality of cylinder banks, each cylinder bank being associated with an exhaust bank, wherein each exhaust bank is equipped with a sensor for monitoring air-fuel ratio, the method including the steps of; applying a cam profile switching signal to a first cylinder bank, prior to an anticipated cam profile switching event on said first cylinder bank, taking a first reading of air-fuel ratio values in each exhaust bank and computing a first relationship X1 between the values, subsequent to the anticipated cam profile switching event on said first cylinder bank, taking a second reading of air-fuel ratio values in each exhaust bank and computing a second relationship X2 between the values, computing a third relationship X3 between X1 and X2, comparing X3 with a predetermined range and if X3 falls within the predetermined range, deducing that a degradation exists in the cam profile switching system of the first cylinder bank and generating a degradation signal.
If X3 falls outside the predetermined range, then this indicates that the cam profile switching system of the switched bank is functioning correctly.
The relationships X1, X2, X3 between values may be a ratio, for example, or may be a difference.
The description has the advantage over the known systems mentioned above in that detection of cam profile switching degradation is reliant only upon lambda sensor differences and not dependent on a problem also being identified with air-flow. Further, it only requires that the assessment takes place across cam profile switching events. In contrast, known systems require continuous monitoring. The description is thus more robust and easier to calibrate than other known systems. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.